Field of the Invention
This invention relates to location-specific critical dimension (CD) alteration/correction flows and processes for improvement of CD uniformity.
Description of Related Art
Techniques disclosed herein relate to microfabrication and, in particular, relate to photolithography and patterning processes. In material processing methodologies (such as photolithography), creating patterned layers typically involves the application of a thin layer of radiation-sensitive material, such as photoresist, to a surface of a substrate. This radiation-sensitive material is transformed into a patterned mask that can be used to etch or transfer a pattern into an underlying layer on a substrate. Patterning of the radiation-sensitive material generally involves exposure by a radiation source through a reticle (and associated optics) onto the radiation-sensitive material using, for example, a photolithography system. This exposure creates a latent image or pattern within the radiation sensitive material which can then be developed. Particular wavelengths of light cause exposed portions of the radiation-sensitive material to change its solubility by either becoming soluble or insoluble to a particular solvent. Developing refers to dissolving and removing a portion of the radiation-sensitive material to yield a topographic or physical pattern, that is, a relief pattern. For example, developing can include removal of irradiated regions of the radiation-sensitive material (as in the case of positive photoresist), or non-irradiated regions (as in the case of negative resist) using a developing solvent. The relief pattern can then function as a mask layer for subsequent processing.
As industry shrinks continue to push minimum feature sizes to smaller and smaller CDs and with the delay and potential high cost of EUV (13.5 nm), the industry has looked for processes that further extend their current ArF (193 nm) immersion (ArFi) scanner systems, including both infrastructure and expertise. CD alteration, such as shrinking/slimming, of the traditional post photolithography ArFi near resolution-limited resist features is one such extension. The ability to improve across-wafer critical dimension uniformity (CDU) around a current CD target, and/or to alter the CD of holes, trenches and/or lines in a controlled process has current and future applications in single patterning, such as in logic design where gate layers have very small features on a slightly less aggressive pitch, and in double patterning/multi-patterning schemes, such as in Litho-Etch-Litho-Etch (LELE) or Litho-Etch repeated “n” times (LEn), Litho-Litho-Etch(LLE), and precursors for sidewall spacers.
The CD alteration process has historically been achieved by 3 methods. The first CD alteration method uses a post-photolithography etch-based plasma trim process for lines (or tapered etch process of holes or trenches), where the process flow includes Coat→Expose→Post Exposure Bake (PEB)→Develop (nominal temperature)→Etch Trim/Shrink. More recently, a second CD alteration method, which is a wet-process, has been proposed in which additional processing steps are performed in the litho-cell, such as a positive tone hot develop (>30° C.) process or an acid rinse/acid rinse bake process, or a combination of the two. The hot develop process shifts the de-protection level at which development stops to a lower level of de-protection. The positive tone hot develop process flow includes Coat→Expose→PEB→Positive Tone Develop (nominal temperature)→Positive Tone Hot Develop (>30° C.). The acid rinse/acid rinse bake process shifts the de-protection level within the matrix of the first developed feature to a higher level, allowing for a second develop process to alter the CD of the feature using standard or modified develop solution. The acid rinse/acid rinse bake process flow includes Coat→Expose→PEB→Positive Tone Develop (nominal temperature)→Acid Rinse→Acid Rinse Bake→Positive Tone Develop (nominal temperature). The combination process flow includes Coat→Expose→PEB→Positive Tone Develop (nominal temperature)→Positive Tone Hot Develop (>30° C.)→Acid Rinse→Acid Rinse Bake→Positive Tone Develop (nominal temperature). Even more recently, a third CD alteration method, which is also a wet-process, has been proposed in which additional processing steps, such as a non-location-specific flood exposure and bake prior to a second development, are utilized to bring the film to a fully or nearly fully de-protected state, at which point development is controlled by development time. The process flow includes Coat→Expose→PEB→Positive Tone Develop (nominal temperature)→Flood Expose→Flood Bake→2nd Develop.
The wet-process examples above are a subset of the various ways in which wet-process CD alteration has been proposed historically.
The first CD alteration method, which is an etch-based plasma method, has the benefit of less potential for pattern collapse due to the lack of any surface tension effects (that are present in wet processing), which means no capillary forces, but has shown the following possible issues that become more problematic at very small CD targets and continued shrinking: the potential to negatively impact or damage organic bottom anti-reflective coatings (BARCs); some secondary effects such as polymer densification that begin to negatively impact structural integrity performance at very small dimensions; pattern density effects, i.e., iso-dense bias; chamber etch uniformity concerns (center-to-edge); process stability/maintainability (due to re-deposition on chamber walls); and/or potential high additional front-end capital cost.
The recently proposed second CD alteration method, which is a wet process, while avoiding etch-specific issues, has the problem of having the magnitude and control of CD change highly correlated to aerial image log-slope (ILS) and resulting de-protection matrix/gradient in the case of the positive tone hot develop process flow.
The other process flows of the second CD alteration method (e.g., the acid rinse/acid rinse bake or the combination process flow), which include the acid rinse and bake steps, similarly come with some new concerns. It is ultimately a diffusion-based process, meaning local amount of CD alteration is correlated to local concentration levels and reaction kinetics as well as time and temperature. Via simulation, it has been observed that this can lead to a potential undercutting due to local changes in the de-protection matrix through defocus and possibly a pattern breaking failure mechanism due to non-homogeneity of resist components leading to stochastic weak points in the line.
The third CD alteration method, which is also a wet process, where the process flow includes the blanket flood exposure and flood bake, similarly comes with some new concerns. Because it attempts to take the film to a fully de-protected state (for homogeneity benefits), it requires modified develop solution conditions to ensure process control via develop time.
Historically, wet CD alteration-based concepts revolved around methods in which the time and/or concentration of a wet chemistry development was linked with the amount and control of CD alteration. Furthermore, to maintain profile control while maximizing the CD alteration amount achievable under these additional development processing steps (CD alteration amount previously limited by level of de-protection remaining within the resist matrix from the patterning exposure), the resist matrix was attempted to be taken to a more homogenized state by introducing methods to increase the de-protection level, if not fully de-protect the resist matrix, e.g., by blanket flood, thermal acid generators (TAGs), and acid rinses.
The condition of a fully de-protected resist matrix prior to the slimming/shrinking develop step (i.e., at the 2nd develop) generally meant that top-loss would be equivalent to side loss. Furthermore, it meant that the develop chemistry had to be altered, for example, using a negative tone develop (NTD) process and developing at the develop rate minimum (Rmin), using a dilute aqueous base developer in a positive tone develop (PTD) process and developing at a modified develop rate maximum (Rmax), using an inhibited aqueous base developer in a PTD process and developing at an inhibited Rmax, and/or using a cold aqueous base developer in a PTD process and developing at a modified Rmax, to make the CD alteration rate reasonable (e.g., 0.1 to a few nm/s) without completely washing away a feature in the first few milliseconds of the 2nd develop. Similarly, the de-protection matrix pre-slimming develop condition (2nd develop) left by acid rinse diffusion and bake generally meant that top-loss would be equivalent to side loss.
There is thus a need for a method to maximize the amount of CD alteration achievable while allowing for more standard development conditions.